Yes, Tech, you are right that NYC is the reason DL is able to expand to some of the off the wall places it is flying. You are right that NYC is a huge market - not unlike LAX and SFO for Asian traffic. I'm also quite sure that the reason DL is pushing to assert itself in the larger NYC market (including transcons and Latin America) is because they know that you cannot rely solely on picking off the ethnic accounts in a couple off the wall destinations to survive. DL recognizes what made CO work by growing to such dominance in NYC (see I can use that word about other airlines) and they want to have a shot at it as well.
As for the word "dominance", I'm not sure what the threshold in marketshare would be but if it makes you feel better AND to let you know I am balanced and fair, I do believe UA DOMINATES the US-Asia market. UA not only serves all of the key markets (although NW serves a couple lesser ones as well) but UA does it on a nonstop basis from the US as well as generally w/ service via NRT (although not true of China) often from more than one gateway. UA also has a healthy ASM advantage over NW in the region. In the same way, DL offers more flights to more cities, many of which are served from more than 1 gateway. I would also say that AA dominates Latin America. So, if it please you, UA dominates Asia, AA dominates Latin America, and DL dominates Europe. Obtaining access one way or the other from NYC to LONDON is the last big strategic hurdle DL needs to cross in Europe.